The present invention relates to digital modem that supports fast retrain based on communication profiles.
Many personal computer systems (PC""s) are sold with, or have the ability to add, a communication device such as a digital or analog modem or the like that enables communication with an external network via telephone wires from the home. The most common external network is the Internet, although other networks including broadband networks, public networks and wide area network (WANs) are contemplated. For simplicity, all such external or broadband networks are referred to herein as WANs. Dial-up modem technology is relatively easy to install and comfortable for the end user to use and currently operates up to a common data rate of approximately 56K bits per second (bps). The 56K bps dial-up modem solution provides a significant increase in speed as compared to a 14.4K bps modem, but does not provide a significant speed advantage as compared to a 28.8K bps modem. The actual modem speed achieved is directly proportional to the quality of the channel characteristics of the voice band signal in a given home location. At the present time, none of the Internet Service Providers (ISP) are guaranteeing the actual rate for the end user.
One significant problem with analog modems are that they interfere with or otherwise cannot be used simultaneously with standard telephones operating according to the Plain Old Telephone Service (POTS) or Message Telecommunications Services (MTS) and other voice band services. The home user is forced to install an entirely separate telephone line in order to have simultaneous access to a WAN while maintaining standard telephone service. An entirely separate and independent phone line is relatively costly and inconvenient.
In spite of advances in dial-up technology, the explosive popularity of the Internet has caused consumers to become frustrated by the relatively slow speed of their connection using analog modems. The Integrated Services Digital Network (ISDN) service has been used at some home sites and small businesses and offers somewhat higher speed access. However, access charges for ISDN are still high for the average consumer and it is not available everywhere. Also, each customer""s loop must first be qualified in order to be connected. Digital Subscriber Loop (DSL) technology, particularly the asymmetrical DSL (ADSL), is beginning to emerge as the technology of choice to provide broadband access to consumers. A digital class of modems supporting ADSL provides significantly higher speed than either analog modems or ISDN devices. Furthermore, ADSL does not require a different type of transmission media such as an ISDN line or the like. Traditionally, however, ADSL required a second phone wire to be installed within the home that required a truck roll and possibly a POTS splitter to be installed at the demarcation point at the home.
The LAN and broadband access issues are also applicable to the small office environment. Small offices commonly include an entirely separate telephone network, a separate LAN network and a separate WAN or broadband connection through a separate physical medium connection such as an ISDN line or the like.
One issue with using ADSL and POTS on the same transmission media is that each phone handset changes the line quality and communication characteristics of the transmission media, such as the characteristic impedance, when on or off hook. Such change in the communication characteristics of the transmission media effects ADSL operation, such as the signal quality, divergence of equalizers, echo cancelers, carrier and timing recovery and other digital signal processing (DSP) algorithms, etc.
It is desired to use existing phone lines and to further provide broadband access without the requirement of a local POTS splitter or separate phone lines. It is further desired to use standard telephone services on the same transmission medium such as a standard phone line without interfering with ADSL communications.
A digital modem that supports fast retrain based on communication profiles according to the present invention includes a memory that stores a plurality of communication profiles. The modem also includes communication logic that sends and receives information via a transmission medium and that operates according to any one of the plurality of communication profiles, and monitor logic that continuously monitors communication by the communication logic on the transmission medium according to any one of the plurality of communication profiles and that provides a signal change indication if the communication characteristics change by at least a predetermined amount. The modem further includes fast retrain logic that selects another one of the communication profiles stored in the memory in response to the signal change indication, that retrains the communication logic to operate according to the selected communication profile and that cooperates with the communication logic to transmit a fast retrain indication via the transmission medium.
In the embodiments illustrated, the digital modem is an ADSL modem that operates according to the Discrete MultiTone (DMT) modulation method using a plurality of upstream and downstream tones, where each tone is within one of a plurality of sub-channels or sub-frequencies of the transmission medium bandwidth. The ADSL modem may operate at a home location without requiring a POTS splitter. The ADSL modem, otherwise referred to as the Remote ADSL Termination Unit (ATU-R), communicates with a Central Office ADSL Termination Unit (ATU-C) located at the central office of the telephone company. Because a POTS splitter is not present, the digital modem operates on the same phone lines or transmission medium as one or more standard telephones operating according to POTS. In this manner, POTS signaling and on/off hook transitions of the telephones change the communication characteristics of the phone lines and affect ADSL communications. The signal quality is compromised so that the same data rate may not be maintained. However, communication may be continued at a lower data rate according to a different profile. The digital modem includes the fast retrain logic to automatically switch between communication profiles to resume communications very quickly.
The digital modem may include measuring logic that cooperates with the communication logic to measure the communication characteristics of the transmission medium and that generates and stores a corresponding communication profile in the memory. In at least one embodiment, each of the communication profiles are measured and stored in this manner. Upon initialization of the modem, the measuring logic is employed to measure the initial signal line quality, which typically reflects the optimal conditions of the phone lines while all of the telephones are on-hook. During operation, the monitoring logic detects a disturbance of the communications, such as when any one or more of the phones transition to off-hook. If the communication profile is no longer valid where the communication logic is unable to sustain communications, and if the fast retrain logic determines that none of the profiles stored in the memory are valid for the given status of the transmission medium, then the measuring logic is again invoked to measure the communication characteristics and generate a new communication profile. The new profile is stored in the memory and sent to the other modem via the transmission medium. Both modems retrain according to the new communication profile. Operation continues in this manner for any number or up to a maximum number of profiles.
The monitoring logic also detects when a disturbance is removed and the communication characteristics improve. If so, the monitoring logic indicates to the fast retrain logic, which finds a better communication profile and quickly retrains the communication logic according to the better communication profile. Also, once all of the likely disturbances are detected and measured and corresponding profiles are generated and stored, the fast retrain logic is able to quickly find the appropriate profile and retrain the modem accordingly with little or no interruption in communications.
In one embodiment, the digital modem measures the transmission medium and stores only two different profiles including an initial best-case profile and a worst-case profile. If a new disturbance causes a greater depreciation of line quality than the current worst-case profile, then a new worst-case profile is generated which replaces the current worst-case profile. The use of only two profiles simplifies the fast retrain indication. Upon receiving the fast retrain indication, both modems simply switch to the other stored profile. In the two profile basic embodiment, the fast retrain indication may comprises a single tone generated on any one of the sub-channels. In the more complex embodiment in which a plurality of profiles are generated and stored, the fast retrain indication may comprise a plurality of tones that identify any one of the profiles. For example, the plurality of tones may implement a binary index to the plurality of communication profiles. In this manner, the remote modem is able to quickly determine the appropriate profile and retrain accordingly.
A modem system according to the present invention includes a transmission medium, a first digital modem implemented in a similar manner described above, and a second digital modem that also includes communication logic, fast retrain logic and memory for storing profiles. The second modem either receives the fast retrain indication and retrains its communication logic to operate according to the indicated communication profile stored locally, or receives and stores a new communication profile and retrains its communication logic to operate according to the new communication profile. In an alternative embodiment, the modems exchange the initial profile and independently calculate and store one or more other profiles based on the initial profile and one or more predetermined cutback value. The predetermined cutback values, for example, may comprise a certain signal-to-noise (SNR) reduction of one or more of the sub-channels.
A method of fast retraining first and second digital modems communicating across a transmission medium according to the present invention includes the first modem measuring the communication characteristics under two different signal conditions of the transmission medium, generating corresponding first and second communication profiles and sending the first and second communication profiles to the second modem via the transmission medium. The method further includes the first and second modems operating and communicating via the transmission medium according to either one of the first and second communication profiles. When the first modem detects a change of the communication characteristics of the transmission medium, it sends a fast retrain indication to the second modem, and the first and second modems switch and operate according to the other communication profile.
The method may further include the first modem, after detecting a change of the communication characteristics of the transmission medium, measuring the communication characteristics of the transmission medium and generating a third communication profile. The first modem then sends the third communication profile to the second modem via the transmission medium. The method may further include the first and second modems operating and communicating via the transmission medium according to any one of the first, second and third communication profiles. When the first modem detects a change of the communication characteristics of the transmission medium, it selects another profile and sends a fast retrain indication to the second modem identifying the selected communication profile. The first and second modems then switch and operate according to the selected communication profile. The method may further include the first modem measuring the communication characteristics under a plurality of different signal conditions of the transmission medium and generating a corresponding plurality of communication profiles. The first modem then sends the plurality of communication profiles to the second modem via the transmission medium. The first and second modems operate and communicate via the transmission medium according to any one of the plurality of communication profiles. If the first modem detects a change of the communication characteristics of the transmission medium, it selects a communication profile other than the current communication profile and sends a fast retrain indication to the second modem that identifies the selected communication profile. Then, the first and second modems switch and operate according to the selected communication profile. The fast retrain indication may comprise a plurality of tones to form an index to the plurality of profiles.
It is now appreciated that a digital modem and modem system according to the present invention enables efficient ADSL communications for a home location without a POTS splitter. Disturbances in the line or signal quality are detected and measured and a new profile is generated and exchanged between the modems. Both modems adapt to the new communication characteristics by retraining according to the new communication profile. At least two profiles, including a best-case profile and a worst-case profile, enable communications to continue with little or no interruption. When operating according to one of two profiles, if one of the modems detects a disturbance or removal of a disturbance, it sends a fast retrain indication and both modems switch to operate according to the other profile. A plurality of profiles may also be generated and used, where one modem determines the proper profile and asserts the fast retrain indication to identify the appropriate profile. In this manner, both modems quickly retrain according to the same profile and resume communications.